Compute the Gibbs free energy change from enthalpy, entropy and temperature, with spontaneity, the crossover temperature and the equilibrium constant K.
kJ/mol
J/(mol·K)
K
Results
Calculated
ΔG
—
kJ/mol, ΔH − TΔS
Spontaneity
—
At the temperature entered
Crossover temperature
—
K, where ΔG = 0
Equilibrium constant K
—
K = e^(−ΔG/RT)
Ready
Enter enthalpy (kJ/mol), entropy (J/mol·K) and temperature (K), then press Calculate.
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What this calculator finds
The Gibbs free-energy change ΔG decides whether a reaction or physical change can proceed spontaneously at constant temperature and pressure. Enter the enthalpy change, the entropy change and the temperature, and this calculator returns ΔG, whether the process is spontaneous, the temperature where the sign flips, and the equilibrium constant that ΔG implies.
It is designed for general chemistry and thermodynamics work, such as checking how heating a reaction changes its spontaneity.
The equations
ΔG = ΔH − TΔS, with ΔH in kJ/mol, ΔS in J/(mol·K) converted to kJ, and T in kelvin.
Tc = ΔH / ΔS, the temperature at which ΔG = 0 (only meaningful when ΔH and ΔS have the same sign).
K = e−ΔG / RT, with R = 8.314 J/(mol·K).
Worked example
Ammonia synthesis N2 + 3 H2 → 2 NH3 has ΔH = −92.2 kJ/mol and ΔS = −198.8 J/(mol·K); use T = 298.15 K, the defaults.
The entropy term is TΔS = 298.15 × (−198.8) / 1000 = −59.27 kJ/mol, so ΔG = −92.2 − (−59.27) = −32.93 kJ/mol. The reaction is spontaneous. Because both ΔH and ΔS are negative, the crossover temperature is (−92.2 × 1000) / (−198.8) = 463.8 K: above that, the unfavorable entropy term overtakes the enthalpy term. The corresponding K is e32,928 / (8.314 × 298.15) ≈ 5.87 × 105.
Common mistakes and how to read the result
Unit mismatch. ΔS in J and ΔH in kJ is the usual convention; check before entering.
Celsius temperatures. Use kelvin. 25 °C is 298.15 K.
Assuming constant ΔH and ΔS. Over wide temperature ranges both drift, so a far-off crossover temperature is only an estimate.
Confusing spontaneous with fast. ΔG says nothing about rate.
Frequently Asked Questions
Why must entropy be converted to kJ?
Enthalpy is normally listed in kJ/mol while entropy is in J/(mol K). The calculator divides T times Delta S by 1000 so both terms are in kJ/mol before subtracting. Entering entropy in kJ by mistake makes the temperature term 1000 times too small.
What does a negative Delta G tell me?
A negative Gibbs free-energy change means the process can proceed spontaneously at that temperature and pressure. It says nothing about speed: a spontaneous reaction can still be extremely slow without a catalyst.
What is the crossover temperature?
When Delta H and Delta S have the same sign, Delta G changes sign at T = Delta H / Delta S. Below that temperature the enthalpy term wins; above it the entropy term wins. If the signs differ, Delta G never changes sign.
Is the value standard-state Delta G?
Only if you enter standard enthalpy and entropy values. The result then equals the standard free-energy change at the chosen temperature, assuming Delta H and Delta S do not vary with temperature.
Gibbs Free Energy Calculator is most useful when the inputs reflect the situation you are actually planning around, not a best-case estimate. Treat the result as a decision aid: it gives you a structured way to compare assumptions, spot outliers, and decide what to verify next. For Chemistry work, the most important review lens is units, concentration, limiting assumptions, temperature, precision, and significant figures.
Start with a baseline run using values you can defend. Then change one assumption at a time and watch which output moves the most. If one input dominates the result, spend your verification time there first. If several inputs have similar influence, use a conservative scenario and an optimistic scenario to create a practical range instead of relying on a single exact number.
Before acting on the result, verify inputs against lab notes, reagent labels, and the expected reaction or solution model. This is especially important when the calculator supports a purchase, project plan, performance target, or operational decision. The calculator can make the math consistent, but the quality of the conclusion still depends on current data, clear units, and assumptions that match your real constraints.
When the output looks surprising, slow down and inspect each input in order. A small change in one high-leverage field can move the final number more than several low-leverage fields combined. For Gibbs Free Energy Calculator, that means you should first confirm the value with the greatest scale, then confirm the value with the greatest uncertainty, then rerun the calculator with conservative and optimistic assumptions. This sequence turns the calculator from a single answer into a practical decision range.
Review Checklist
Confirm every input uses the unit and time period requested by the calculator.
Run a low, expected, and high scenario so the answer has a useful range.
Check whether rounding or a missing decimal place changes the decision.
Update the calculation for every new mixture, batch, reaction, or homework data set.
How to Validate the Result
Use Gibbs Free Energy Calculator as a repeatable checkpoint rather than a one-time answer. The safest workflow is to record the original inputs, save the output, and write down which assumption you are testing. Then rerun the calculator with one changed value. If the result changes sharply, that input deserves more attention before you act on the number.
For this topic, the main validation lens is units, concentration, limiting assumptions, temperature, precision, and significant figures. That means a result can be mathematically correct and still be misleading if the inputs come from the wrong time period, use inconsistent units, or mix expected values with best-case values. Keep baseline, conservative, and optimistic runs separate so the final decision is easier to explain later.
When you share the result with someone else, include the assumptions and the date of the calculation. Many calculator outputs become stale after prices, schedules, measurements, or constraints change. A short note about the source of each input makes the calculation auditable and prevents later confusion about why the answer moved.
Label the source for each input before comparing scenarios.
Use the same rounding method across every run.
Flag any input that is estimated rather than measured.
Recalculate for every new mixture, batch, reaction, or homework data set.